I had a question from my lovely and understanding girlfriend the other day. While I was trying to explain how Wi-Fi works. She asked the perfect question why is it measure it on a negative scale. I why is it -65dBm good. I struggled to give an answer that she understood. This was because since studying my CWNA a few years back this information wasn't fresh in my mind and the recall was poor, to be honest. So I thought I write this blog to have an easy recall place for important information
So first to start why is it measured in dBm in the first place. The reason is to make the numbers easier. If we just used milliwatt it would be hard to compare the difference in power. For example 0.0000000001 mW to 0.00001mW, or 5 orders of magnitude difference.
So using dBm makes the math much more simple. Signal strength is measured in dBm is because a logarithmic scale is much easier for people to comprehend. When we measure in dBm, we are using a measurement where 1 mW (Milli Watt) of power is defined as 0 dBm. Measured power below 1 mW is expressed as negative. Power above 1 mW is positive. When measuring this way, every 10 dBm that you increase is 10x the power. Every 10 dBm of decrease is 1/10th the power.
0 dBm = 1 mW
10 dBm = 10 mW
20 dBm = 100 mW
30 dBm = 1000 W = 1 W
-10 dBm = 0.1 mW
-20 dBm = 0.01 mW
It also allows us to easily define SNR or Signal to Noise Ratio. A dBm measurement by itself does not tell you everything that you need to know. If a received signal is -65 dBm and the noise floor is -66 dBm, communications will be difficult if not impossible. The SNR, in this case, would be 1 dB. If the received signal is -85 dBm and the noise floor is -100 dBm, communications can easily be had. The SNR in this case is 15 dB, which is quite good. SNR is a ratio, so it is unitless and the negative values will cancel in this case. But since we are using a logarithmic scale, you can simply note the difference in dBm received from the dBm of the noise floor to determine the value.
So, in conclusion, the reason is that because the power actually used in producing Wi-FI signal is less than 1 MW and to make it easier to see the difference in the signal we use a logarithmic scale to make it simpler to understand the difference between different signal levels and makes calculating importing things like SNR a whole lot easier.
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